1. Field of the Invention
The invention relates to a process of, and apparatus for, treating sewage while it is flowing through a sewer.
2. Description of the Prior Art
Problems can arise in conveying sewage through gravity sewers as a result of the action of facultative bacteria present in the sewage. These bacteria normally respire oxygen dissolved in the sewage, but if this supply of oxygen is exhausted, they obtain their oxygen for respiration by reducing compounds containing oxygen, particularly sulfates.
This bacterial reduction of sulfates yields hydrogen sulfide, a malodorous, poisonous gas. Sometimes lethal concentrations of hydrogen sulfide are created at locations that are accessible to maintenance engineers, and there is frequently escape of hydrogen sulfide from the sewer to above ground. Formation of hydrogen sulfide is particularly marked under hot climatic conditions, since these conditions promote bacterial activity.
Another problem attributable to hydrogen sulfide is the corrosion of gravity sewers. This is caused by aerobic bacteria oxidising hydrogen sulfide to sulfuric acid on the damp walls of a gravity sewer in the air space above the level of the sewage.
Various methods have been tried in order to prevent the bacterial activity which forms hydrogen sulfide. For example, bacterial activity can be inhibited by the addition of gaseous chlorine, but this can result in the sterilization of the sewage. Moreover, chlorine itself is both highly corrosive and toxic.
It is possible to freshen sewage by adding air. With air, however, once a concentration of dissolved oxygen of 10 ppm has been attained, no more oxygen will dissolve. Furthermore, the sewage is usually saturated with nitrogen before the air is introduced. Accordingly, none of the nitrogen contained in the air is dissolved. This undissolved nitrogen can cause gas locks and other hydraulic problems in the sewer.
Proposals have been made to introduce pure oxygen into sewage being treated in the secondary stage of an activated sludge plant. Such a process is described in U.S. Pat. No. 3,607,735. But no proposal has been made to inject an oxygen-rich gas into a sewer.